[Tagging] Power generation refinement: power plant model evolution

[Alberto]

Thanks François for your great work.
One thing: in "PV solar panel" example, you should change
generator:type=solar_panel to generator:type=solar_photovoltaic _panel, for
consistency with the rest of the proposal.
+1 for relations: as written in the proposal, "this advanced tagging is
optional, it is allowed to ensure consistency with wind farms, and ease the
computing of total power plant rating, by adding all the generator:output:*
values of its generator members".
It is useful for wind farms, solar fields, hydro dams, and anywhere the
power elements are dispersed and without a physical perimeter.
Bye
Viking81 - Alberto